Rigid polyurethane foam in general is a closed cell structure entrapping a gas such as a halogenated hydrocarbon or a chlorofluorocarbon, e.g. trichlorofluoromethane (hereinafter referred to briefly as R-11), or carbon dioxide. Since such gases are of low thermal conductivity, the closed cell polyurethane foam has an excellent heat insulating property. Moreover, rigid polyurethane foam has good formability in addition, so that it has been used as heat insulators and structural members in a broad field.
However, the conventional chlorofluorocarbons represented by R-11 are chemically so stable that they diffuse undecomposed into the stratosphere to destroy the ozone layer, thus presenting a serious threat to the earth ecology. For this reason, the use of chlorofluorocarbons has been increasingly restricted in recent years, with a complete ban being foreseen in the near future. Therefore, a great deal of research is being undertaken for developing blowing agents that may substitute chlorofluorocarbons. So far, 1,1-dichloro-1-fluoroethane (hereinafter referred to briefly as HCFC-141b) and methylene chloride, among others, have been proposed as possible substitutes for R-11.
However, the thermal conductivity of a closed cell rigid polyurethane foam depends on the thermal conductivity of the blowing gas employed. Therefore, a heat insulating material of low thermal conductivity can hardly be obtained using a substitute so far proposed, such as HCFC-141b which is definitely greater in thermal conductivity than R-11. Furthermore, any closed cell rigid polyurethane foam suffers significant dimensional changes, deformations and curling in a widely fluctuating temperature environment and is, therefore, limited in the scope of application.
In view of the above situation, a heat insulating material, called "evacuated heat insulator", which comprises a foam core encased in a gas barrier material, evacuated and sealed gas-tight has been proposed. The core heretofore known for such a heat insulating material includes inorganic substances, such as perlite, typically in a powdery form or in a honeycomb form. However, any heat insulating material utilizing an inorganic core material such as perlite is not only poor in manufacturing workability but also of high density and costly.
Heat insulating materials utilizing a core of organic materials, such as an open cell rigid poly urethane foam, have also been proposed (JP Kokai S-57-133870, JP Publication H- 1-4112) . Such an insulating material is generally manufactured by covering an open cell rigid polyurethane foam core with a gas barrier casing material, evacuating the internal cells and sealing the structure. However, the cell size such core is 300-1000 .mu.m and in order to insure a sufficient heat insulating performance, it takes a long evacuation time to reduce the internal pressure to the order of 0.001 mmHg. This is a considerable productivity drawback and, therefore, the technology is not suited for mass production. Furthermore, when such a core of polyurethane foam is employed, the insulating material cannot be tailored to the irregular wall surface of an electric refrigerator, for instance, but can be applied only to flat surfaces.